Maybe there is already some tool that does that, but you can also create a simple script with some screenshot tool and tesseract, as you are trying to use.

Take as an example this script (in my system I saved it as /usr/local/bin/screen_ts):

#!/bin/bash

# Dependencies: tesseract-ocr imagemagick scrot



select tesseract_lang in eng rus equ ;do break;done

# Quick language menu, add more if you need other languages.



SCR_IMG=`mktemp`

trap "rm $SCR_IMG*" EXIT



scrot -s $SCR_IMG.png -q 100 

# increase quality with option -q from default 75 to 100

# Typo "$SCR_IMG.png000" does not continue with same name.





mogrify -modulate 100,0 -resize 400% $SCR_IMG.png 

#should increase detection rate



tesseract $SCR_IMG.png $SCR_IMG &> /dev/null

cat $SCR_IMG.txt

exit

And with clipboard support:

#!/bin/bash 

# Dependencies: tesseract-ocr imagemagick scrot xsel



select tesseract_lang in eng rus equ ;do break;done

# quick language menu, add more if you need other languages.



SCR_IMG=`mktemp`

trap "rm $SCR_IMG*" EXIT



scrot -s $SCR_IMG.png -q 100    

# increase image quality with option -q from default 75 to 100



mogrify -modulate 100,0 -resize 400% $SCR_IMG.png 

#should increase detection rate



tesseract $SCR_IMG.png $SCR_IMG &> /dev/null

cat $SCR_IMG.txt | xsel -bi



exit

It uses scrot to take the screen, tesseract to recognize the text and cat to display the result. The clipboard version additionally utilizes xsel to pipe the output into the clipboard.

NOTE: scrot, xsel, imagemagick and tesseract-ocr are not installed by default but are available from the the default repositories.

You may be able to replace scrot with gnome-screenshot, but it may take a lot of work. Regarding the output you can use anything that can read a text file (open with Text Editor, show the recognized text as a notification, etc).

GUI version of the script

Here's a simple graphical version of the OCR script including a language selection dialog:

#!/bin/bash

# DEPENDENCIES: tesseract-ocr imagemagick scrot yad

# AUTHOR:       Glutanimate 2013 (http://askubuntu.com/users/81372/)

# NAME:         ScreenOCR

# LICENSE:      GNU GPLv3

#

# BASED ON:     OCR script by Salem (http://askubuntu.com/a/280713/81372)



TITLE=ScreenOCR # set yad variables

ICON=gnome-screenshot



# - tesseract won't work if LC_ALL is unset so we set it here

# - you might want to delete or modify this line if you 

#   have a different locale:



export LC_ALL=en_US.UTF-8



# language selection dialog

LANG=$(yad 

    --width 300 --entry --title "$TITLE" 

    --image=$ICON 

    --window-icon=$ICON 

    --button="ok:0" --button="cancel:1" 

    --text "Select language:" 

    --entry-text 

    "eng" "ita" "deu")



# - You can modify the list of available languages by editing the line above

# - Make sure to use the same ISO codes tesseract does (man tesseract for details)

# - Languages will of course only work if you have installed their respective

#   language packs (https://code.google.com/p/tesseract-ocr/downloads/list)



RET=$? # check return status



if [ "$RET" = 252 ] || [ "$RET" = 1 ]  # WM-Close or "cancel"

  then

      exit

fi



echo "Language set to $LANG"



SCR_IMG=$(mktemp) # create tempfile

trap "rm $SCR_IMG*" EXIT # make sure tempfiles get deleted afterwards



scrot -s "$SCR_IMG".png -q 100 #take screenshot of area

mogrify -modulate 100,0 -resize 400% "$SCR_IMG".png # postprocess to prepare for OCR

tesseract -l "$LANG" "$SCR_IMG".png "$SCR_IMG" # OCR in given language

xsel -bi < "$SCR_IMG".txt # pass to clipboard

exit

Aside from the dependencies listed above you will need to install the Zenity fork YAD from the webupd8 PPA to make the script work.